Antitoxins act to neutralize the effects of toxins on specific cells; production of a specific level of protective antitoxins or antibodies following immunization with toxoid vaccines containing a Diphtheria component is required for release of the vaccine lot. Currently, a second animal test is employed to evaluate the efficacy of the animal response to the injected vaccine. This research is focused on replacing this second animal test with an in vitro cell test. This year, a major collaborative study involving 6 participating laboratories was conducted to evaluate the results of the Vero Cell assay with the animal assay in each laboratory. All participating laboratories concurred on the results of the samples manipulated to fail or pass. Data analysis is continuing to define acceptable limits for the Vero assay and correlated these values to the historically based confidence limits for the animal test.